1. Field of the Invention
This invention relates generally to optical apparatus in semiconductor technology, and more particularly, to a test monitor for use in focusing an image on a semiconductor wafer.
2. Discussion of the Related Art
Typically, an optical system 30 (FIG. 1) used for patterning photoresist 32 on a semiconductor wafer 34 comprises a light source 36, a mask or reticle 38 having opaque lines 40 and transparent portions 42, and a lens 44, the light from the light source 36 passing through the transparent portions 42 of the mask/reticle 38 through the lens 44 and to the photoresist 32, with light being blocked from reaching the lens 44 (and photoresist 32) by the opaque lines 40 of the 38 mask/reticle.
As is well known, there is a need to position the wafer 34 at a proper distance with respect to the lens 44 so that images that fall on the photoresist 32 of the wafer 34 will be in the plane of best focus.
Typically, prior to actual fabrication of semiconductor devices, a test focus monitor in the form of for example a reticle is used as part of the overall system to achieve proper focus of the image on the wafer. An example of such a monitor is shown and described in the paper entitled xe2x80x9cNew Phase Shift Gratings For Measuring Aberrationsxe2x80x9d, by Hiroshi Nomura, published by SPIE, dated Feb. 27, 2001, which is herein incorporated by reference. FIGS. 2-4 herein show a monitor 50 configured as shown in FIGS. 3 and 5 of that paper. The monitor 50 is made up of a quartz base 52 which is transparent to light, and which has a plurality of parallel, opaque, spaced apart lines 54 on the base 52, the lines 54 having a first set of adjacent ends 55, and a second, opposite set of adjacent ends 56. The area between each adjacent pair of lines 54 is transparent to light and is made up of regions 58 which pass light therethrough without changing the phase thereof, and regions 60 which pass light therethrough which shift the phase of such light by 90xc2x0 (the phase shifting caused by recesses 62 in the base 52xe2x80x94FIGS. 3 and 4 and the above cited paper). Each of the lines 54 has a region 58 and a region 60 which are aligned along and on one side thereof, and a region 58 and a region 60 which are aligned along and on the opposite side thereof. Each of the lines 54 has a region 58 on one side thereof opposite a region 60 on the other side hereof, these regions 58, 60 running from end 55 of that line to adjacent to the middle thereof, and furthermore, each of the lines 54 has a region 60 on the one side thereof opposite a region 58 on the other side thereof, these regions 60, 58 running from end 56 to adjacent the middle thereof.
FIGS. 3 and 4 are views similar to that shown in FIG. 1, but incorporating the monitor 50 of FIG. 2 as a part of the system 30. FIG. 3 includes a sectional view of the monitor 50 taken along the line 3xe2x80x943 of FIG. 2, showing a cross-section of the upper area 50A of the monitor 50, wile FIG. 4 includes a sectional view of the monitor 50 taken along the line 4xe2x80x944 of FIG. 2, showing a cross-section of the lower area 50B of the monitor 50. As will be seen, with reference to the upper area 50A of the monitor 50 (FIG. 3), moving the wafer 34 and lens 44 relatively together and apart causes the shadows 64A, 64B, 64C formed on the photoresist 32 of the wafer 34 (formed by the opaque lines 54) to shift (downward as the wafer 34 and lens 44 are moved relatively further apart). Meanwhile, with reference to the lower area 50B of the monitor 50 (FIG. 4), moving the wafer 34 and lens 44 relatively together and apart causes the shadows 64D, 64E, 54F formed on the photoresist 32 of the wafer 34 to shift (upward as the wafer 34 and lens 44 are moved relatively further apart). The dotted lines 66 in FIGS. 3 and 4 indicate the traverse of the shadows 64A, 64B, 64C, 64D, 64E, 64F as the wafer 34 is so moved relatively toward and away from the lens 44.
These paths are plotted in FIG. 5, and the intersections thereof indicate the best focus of the image on the wafer 34.
FIG. 6 includes FIGS. 6A-6F which are views taken along the lines 6Axe2x80x946A, 6Bxe2x80x946B, 6Cxe2x80x946C, 6Dxe2x80x946D, 6Exe2x80x946E, and 6Fxe2x80x946F of FIGS. 3 and 4. With the wafer 34 and lens 44 closest together as shown in FIGS. 3 and 4, FIGS. 6A and 6D show the simultaneous positions of the shadows 64A-64F on the photoresist 32 determined by the respective areas 50A, 50B of the monitor 50 With the wafer 34 and lens 44 so positioned relative to each other, the photoresist 32 is exposed to light from the light source 36 and is then developed to determine photoresist lines, which corresponds to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6A and 6D are misaligned As the wafer 34 and lens 36 are moved relatively further apart to an intermediate position as shown in FIGS. 3 in 4, FIGS. 6B and 6E show the simultaneous positions of the shadows 64A-64F on the photoresist 32 determined by the respective areas 50A, 50B of the monitor 50. Again, the photoresist 32 is exposed to light from the light source 36 and is then developed to determine photoresist lines that correspond to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6B and 6E are substantially in alignment. Then, as the wafer 34 and lens 44 are moved relatively further apart, i.e., to their most far apart positions as shown in FIGS. 3 and 4, FIGS. 6C and 6F show the simultaneous is positions of the shadows 64A-64F on the photoresist 32 determined by their respective areas 50A, 50B of the monitor 50. Again, with the wafer 34 and lens 44 so positioned relative to each other, the photoresist 32 is exposed to a light from the light source 36 and is then developed to determine photoresist lines, which correspond to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6E and 6F are misaligned.
It will be seen that changing the distance between the lens 44 and wafer 34 causes the shadows 64 A-64C to move further in and out of alignment with the shadows 64D-64F. The process of moving the lens 44 and wafer 34 relatively closer together and further apart, along with the corresponding exposure and development of the photoresist 32 accompanying each adjustment is repeated until the lines formed in the photoresist 32 are substantially straight. This is illustrated in FIG. 6 of the above cited paper.
While such an approach is useful, only a relatively coarse reading of focus is achievable. For example, with reference to FIG. 6 of the above cited paper, only a small shift in the pattern from top to bottom is shown over a range of 400 nm of relative movement between the wafer 34 and lens 44. With device dimensions continually being reduced, there is a need to achieve a proper reading of focus within a much smaller range of lens-wafer relative movement, for example, 200 nm or less.
Therefore, what is needed is an apparatus which is capable of providing proper focus of an image on a wafer through a very small range of relative movement between a lens and a wafer.
In the present invention, an optical tool is provided, made up of a tool body having a first plurality of parallel, substantially opaque, spaced apart lines thereon, and a second plurality of parallel, substantially opaque, spaced apart lines thereon with a relatively small angle between the first and second pluralities of lines. As an image of the lines of the first plurality thereof is provided on a semiconductor body, such line images move relative to the semiconductor body as the semiconductor body is moved relatively toward and away from the optical tool. Furthermore, as an image of the lines of the second plurality thereof is provided on the semiconductor body, such line images move relative to the semiconductor body as the semiconductor body is moved relatively toward and away from the optical tool, but in a manner different from the movement of the image of the lines of the first plurality thereof. The moirxc3xa9 fringe formed on the semiconductor body from images of the first and second plurality of lines is analyzed in order to achieve proper focus of an image on the semiconductor body.
The present invention is better understood upon consideration of the detailed description below, in junction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.